Spot potato blight early with our guide to Phytophthora infestans. Learn identification, symptoms, lifecycle, and organic-first controls for UK gardens. Protect your spuds from this destructive disease.
Potato blight, caused by the oomycete Phytophthora infestans, is one of the most destructive diseases for potato growers in the UK. This fungus-like pathogen thrives in warm, wet conditions, rapidly spreading to devastate foliage, stems and tubers. It famously triggered the Irish Potato Famine and still causes major losses today. You can lose an entire crop in days if unchecked, but vigilance, resistant varieties and good hygiene keep it in check. Tomatoes are also at risk in your veg plot.
Identification
Look for dark, water-soaked patches on leaves, often starting as small, irregular spots 2-10mm across with pale green halos. These turn brown and papery in dry weather. Flip the leaf: white, fuzzy mould appears on the underside in humid conditions – that's sporangia forming. Stems show greasy, dark streaks; tubers have reddish-brown dry rot beneath the skin, sometimes with white mould. Lesions expand fast in wet spells. Check regularly from June, especially after rain.
Damage & symptoms
Foliage collapses into blackened, wilted masses within days, halting photosynthesis and stunting tubers. Stems rot from lesions, killing upper growth and allowing rain to splash spores to tubers. Infected tubers rot in ground or store, turning firm reddish-brown inside then soft and foul-smelling. Whole plants die rapidly in humid weather. Yields drop sharply; survivors produce small, poor-quality spuds. Defoliation exposes tubers to greening and further blight infection.
Lifecycle
Phytophthora infestans spreads mainly asexually via sporangia, released from leaf/stem lesions in humid conditions. Wind and rain splash these up to 50m, or further on gusts. They germinate in water films above 10°C, infecting new tissue overnight. Tubers harbour overwintering mycelium as primary inoculum. Sexual reproduction (rare in UK) creates hardy oospores in soil. Peaks in warm (15-20°C), wet summers; dies back without living hosts.
Prevention — Choose resistant varieties
Plant blight-resistant potatoes like Sarpo Mira, Setanta or Rooster. These show high foliar and tuber resistance scores from AHDB trials. Check National Trial results annually as strains evolve. Space plants well for airflow.
Prevention — Promote good airflow
Earth up rows promptly and avoid overcrowding. Hill soil covers tubers early, preventing spore splash. Remove lower leaves touching soil. Site in sunny, open spots with shelter from wind-driven rain.
Prevention — Crop rotation and hygiene
Rotate potatoes on a 3-4 year cycle with non-solanaceous crops like brassicas. Dig and bin all diseased foliage immediately – don't compost. Harvest promptly and dry tubers fully before storing cool and dark.
Prevention — Monitor weather risks
Use Blight Watch or apps for local forecasts. Spray preventatively at first high-risk warning. Avoid overhead watering; mulch to suppress soil splash.
Organic control — Copper-based sprays
Apply copper oxychloride or Bordeaux mixture as a protectant every 7-10 days in high-risk periods. Permitted under organic rules; rotate with other methods to avoid resistance. Best before infection starts.
Organic control — Biological agents
Products with Trichoderma or Bacillus subtilis colonise leaf surfaces, outcompeting blight. Apply weekly in warm, humid spells. Combine with seaweed extracts to boost plant defences.
Organic control — Cut and destroy
At first signs, cut off affected foliage and stems, burying deeply or incinerating away from plot. This halts spore spread. Repeat checks daily during outbreaks.
Organic control — Potato-derived extracts
Foliar sprays from resistant potato extracts trigger plant immunity. Apply preventatively from tuber emergence.
Frequently asked questions
Can I still eat blighted potatoes?
Cut away firm, healthy-looking flesh beyond any reddish-brown rot. Discard if rot is widespread or tubers soft/mouldy. Dry and store sound ones cool (4-7°C), dark and ventilated. Peel deeply before cooking. Never eat sprouting or green tubers.
Does blight affect shop-bought potatoes?
Rarely, as they're treated and stored cold. But don't plant them – they may carry dormant blight. Grow certified seed potatoes only.
How do I know if my crop is at risk?
Track local warnings via Blight Watch app or Met Office. Risk builds above 10°C with leaf wetness >7 hours. Inspect weekly from mid-June.
Will blight kill my tomatoes too?
Yes, same pathogen hits tomatoes hard outdoors. Space apart from potatoes; use greenhouse varieties like Mountain Magic if growing both.
Does frost stop blight?
Frost kills sporangia but not tuber mycelium. New cycles start from volunteers or infected stores in spring.